Technologies regarding a radio system provided with a plurality of antennas and a high-frequency unit (hereinafter, such system is referred to as “multi-antenna radio system”) are currently studied and developed eagerly because the multi-antenna radio system is regarded as means for achieving current and future large-capacity, high-speed communications.
In the technological field of the wireless LAN (Local Area Network), the methods especially attracting attention are the Maximum Ratio Combining (MRC) method for improving the reception performance by controlling the directionality of the multi-antenna radio system, and the Multiple Input Multiple Output (MIMO) method for increasing the communication capacity.
The Minimum Mean Square Error (MMSE) method is known as a high-accuracy channel equalizing means in the multi-antenna wireless system, and as a high-accuracy signal separating/combining means in the MIMO method.
The following describes an outline of operations by the MMSE method for generating a weight matrix for use in the weighting calculation performed onto reception signals, and performing the weighting calculation onto the reception signals using the generated weight matrix.
First, in the MIMO method, the radio communication device estimates the channel characteristic for each channel in accordance with pilot signals included in the reception signals, and estimates the noise power included in the reception signals. Then, the radio communication device generates a weight matrix WC by performing a calculation of the following Equation (1) using a channel matrix HC whose matrix elements are estimation values of channel characteristics of each channel obtained by the estimation, and using an estimation value σC2 of the noise power of noise included in each reception signal obtained by the estimation. Note that the weight matrix WC is a matrix whose matrix elements are weighting coefficients for the signals corresponding to the provided antennas.[Equation 1]WC=(HCHHC+σC2I)−1HCH  (1)
In Equation (1), “I” represents a unit matrix, “[ ]H” represents a complex conjugate transposed Matrix of a matrix [ ], and “[ ]−1” represents an inverse matrix of a matrix [ ].
Note that Equation (1) is a calculation equation for obtaining the weight matrix WC by a general MMSE method.
Next, the radio communication device estimates a transmission signal with a maximized ratio of signal power to noise power by performing a calculation of the following Equation (2) using the weight matrix WC obtained by the calculation of Equation (1) above, and using a reception signal matrix rC whose matrix elements are reception signals . Note that in the following Equation (2), “sC” represents a transmission signal or a transmission signal matrix whose matrix elements are transmission signals.[Equation 2]sC=WCrC  (2)
It is understood from the Equations (1) and (2) that, to achieve a high-accuracy estimation of a transmission signal, it is necessary to generate the weight matrix WC accurately, and that, to generate the weight matrix WC accurately, it is necessary to estimate the channel characteristic and noise power of each channel accurately.
In the future radio communications with large-capacity, high-speed transmissions, it will be necessary to estimate the noise power more accurately than now and before. However, as the case now stands, development of technologies for estimating the noise power accurately is inactive.
In these circumstances, there are some documents that disclose methods of estimating the noise power with the MMSE method. The following describes an outline of the technology disclosed in Patent Document 1 identified below, as one example of such documents.
A noise power estimation device disclosed in Patent Document 1 calculates a correlation between a reception signal and a pilot signal, and obtains a reception power for each channel. The noise power estimation device then, with use of a predetermined power ratio between the pilot signal and a data signal, obtains a corrected reception power of the pilot signal for each channel by removing a multipath interference component from the reception power of the pilot signal for each channel.
The noise power estimation device estimates a noise power by estimating a total power of the pilot signal and data signal included in the reception signal based on the corrected reception power of a plurality of channels and the predetermined power ratio, and subtracting the estimated total power from the total power of the reception signal.
Also, Patent Document 2 identified below is one example of documents disclosing a method for preliminarily measuring a plurality of characteristic amounts with respect to interference and noise of unnecessary signals before a desired signal arrives, and when a desired signal arrives, maximizing a ratio of the power of desired signal to the power of unnecessary signals by the MMSE method, using the characteristic amounts of unnecessary signals having been measured preliminarily.
FIG. 26 shows the structure of a radio communication device disclosed in Patent Document 2.
In a radio communication device 1000, antennas 1011 and 1015 receive reception signals, variable gain amplification units 1021 and 1025 amplify the reception signals, and then down converters 1031 and 1035 convert them into reception signals in the baseband. An AGC unit 1040 controls the gain of the variable gain amplification units 1021 and 1025 so that the amplitude level or the power level of the output signal of the down converters 1031 and 1035 becomes constant.
When the characteristic amounts of unnecessary signals are measured (hereinafter referred to as “during measuring of the interference”), an interference noise estimation unit 1060 estimates a plurality of characteristic amounts of unnecessary signals, based on the reception signal in the baseband. The interference noise estimation unit 1060 then generates a covariance matrix RUUC whose matrix elements are estimation values of the characteristic amounts obtained by the estimation, and holds the generated covariance matrix RUUC. Note that the covariance matrix RUUC is a covariance matrix of unnecessary signal matrix UC whose matrix elements are unnecessary signals corresponding to the provided antennas.
Also, during measuring of the interference, the gain value (hereinafter referred to as “interference gain value”) of the variable gain amplification units 1021 and 1025 controlled by the AGC unit 1040 is held by an interference measuring time gain holding unit 1071 provided in an amplitude correction unit 1070.
During reception of a desired signal, a channel characteristic estimation unit 1050 estimates the channel characteristic for each channel based on the pilot signal included in the reception signal in the baseband, and generates a channel matrix HC whose matrix elements are the estimation values of each channel characteristic obtained by the estimation.
Also, during reception of a desired signal, a gain ratio calculation unit 1072 determines a gain value (hereinafter referred to as “desired gain value”) of the variable gain amplification units 1021 and 1025 controlled by the AGC unit 1040, and calculates a gain ratio ΔgC that is a ratio of the desired gain value to the interference gain value held by the interference measuring time gain holding unit 1071. A multiplication unit 1073 then obtains a matrix
ΔgCRUUC by multiplying the gain ratio ΔgC by the covariance matrix RUUC held by the interference noise estimation unit 1060, and outputs the obtained matrix ΔgCRUUC to a weight generation unit 1080.
The weight generation unit 1080 generates a weight matrix WC by performing a calculation of the following Equation (3) using the channel matrix HC and the matrix ΔgCRUUC.[Equation 3]WC=HCH(HCHCH+ΔgCRUUC)−1  (3)
A weighting calculation unit 1090 estimates a transmission signal by multiplying the weight matrix WC by a reception signal matrix rC whose matrix elements are reception signals in the baseband, and a demodulation unit 1100 demodulates an estimated transmission signal.
Patent Document 1: Japanese Patent Application Publication No. 2005-328311
Patent Document 2: International Publication Pamphlet No. 2006/003776